The present invention relates to a differential pressure sensor which is suitable for measuring a differential pressure between two fluids, and more particularly to a multiple function type differential pressure sensor which is capable of measuring even a static pressure and a temperature.
As for a differential pressure sensor for measuring a differential pressure, in particular, a multiple function type differential pressure sensor in which a sensor for measuring a differential pressure, a sensor for measuring a static pressure and a sensor for measuring a temperature are provided on one chip and thus the differential pressure, the static pressure and the temperature can be measured simultaneously, there have been proposed many examples which are disclosed in JP-A-61-240134, and JP-B-2-9704 for example. Each example of the multiple function type differential pressure sensor is designed in such a way that a differential pressure sensor unit as the main sensor includes four resistors which are made of a semiconductor material and are sensitive to a differential pressure, and the four resistors are provided on a part having a relative large wall thickness (thick part) called a pressure sensitive diaphragm. Moreover, on the thick part other than the pressure sensitive diaphragm, several resistors are provided which are made of a semiconductor material and are sensitive to the static pressure (line pressure) and the temperature. Those resistors made of a semiconductor material are formed, together with the above-mentioned resistors made of a semiconductor material, on a semiconductor substrate by the thermal diffusion method or the ion implantation method in the well-known semiconductor manufacturing process. The semiconductor substrate is bonded to a fixing base and the fixing base having the semiconductor substrate bonded thereto is then mounted to a housing. The above-mentioned multiple function type differential pressure sensor of this sort compensates the zero-point-change of the differential pressure sensor, which is due to the line pressure change and the temperature change of the process, by utilizing signals sent from the auxiliary sensors (the sensor for measuring a static pressure, and the sensor for measuring a temperature) provided thereon, thereby to obtain a differential pressure signal at high accuracy.
However, in the above-mentioned examples, in particular, the example disclosed in JP-B-2-9704, the signal for representing a static pressure is obtained by utilizing a bending strain which is due to the difference between a modulus of longitudinal elasticity of the semiconductor substrate and that of the fixing base when applying the static pressure. Therefore, only a compensating signal which has a very small output and a low signal-to-noise (S/N) ratio can be obtained. Moreover, since the bending strain is produced in order to obtain that static pressure signal, the bending strain exerts an influence on the differential pressure sensitive diaphragm of the main sensor, so that the differential pressure signal and the static pressure signal interfere with each other. Therefore, in order to obtain the differential signal at high accuracy, the input-output characteristics of the differential pressure sensor as the data for the compensation need to be finely collected while changing the temperature and the static pressure.
On the other hand, according to the example disclosed in JP-A-61-240134, since the differential pressure and the static pressure, which are the pressures to be detected, are detected through the provision of the respective pressure sensitive parts, the static pressure signal can be obtained which is considerably large as compared with the case of the above-mentioned example. In this example, however, for the purpose of obtaining the signal having a large S/N ratio, it is necessary to provide an introduction line for introducing the reference pressure into a rear surface of a pressure sensitive part for the static pressure signal. Therefore, such an arrangement of the sensor is basically, substantially equal to the new provision of the separate static pressure sensor (pressure sensor). Therefore, the method of assembling a sensor and the manufacturing process become complicated, and as a result, the sensor will be poor in reliability and profitability. If possible, it is desirable from various industrial points of view that the various kinds of sensors are formed on one chip integrally with one another in the manner as in the former example.
As described above, in the multiple function type differential pressure sensors disclosed in the above-mentioned examples, the zero-point-change of the differential pressure sensor as the main strain sensor when applying the static pressure is mainly noticed. Thus, the zero-point-change is compensated using the output signal of the static pressure sensor as a parameter. On the other hand, when applying the static pressure to the differential pressure sensor, not only the change of the zero point as described above but also the change of the span (span change) occurs necessarily. If as the method of compensating the span change, the above-mentioned method of compensating the zero-point-change is used, the compensation data which is obtained by changing the static pressure state and the differential pressure must be collected. As a result, the enormous amount of data will be provided, so that it is difficult to perform the compensation. Therefore, in the prior art differential pressure sensor, if the zero-point-change and the span change are compared with each other, the span change should be regarded as important as compared with the zero-point-change in order to improve the accuracy of the measurement. However, for the span change, the compensation is not performed or the simple compensating method is carried out.